Induction heated furnace



1934- v. J. c. SORREL INDUCTION HEATED FURNACE Filed Deep 15, 1931 2 Sheets-Sheet l INYENTOR J1, JmeL 4' ATTORNEY:

V. J. C. SORREL INDUCTION HEATED FURNACE Filed Dec. 15. 1931 2 Sheets-Sheet 2 4 0 6'0 6'0 70 80 90 mo 00 I;

7 6. 5M slmudvwwkw can i atented ct. 2, i934 assignor corporation of France to Ugine-Infra, Grenoble, France, a

Anplioation December 15, 1931, Serial No. sauce In France January 23, 193i.

4 Claims.

Induction-heated furnaces are already well known, whereof the muiile consists of a magnetic metal or alloy and it is likewise known that a suitable selection of said metal or alloy of the mufie and a suitable adjustment of the furnace characteristics and of the induction current characteristics permit the temperature of said muiile to be maintained automatically at a constant value.

In furnaces of this type already available, the increase of temperature of the mume is partly due to currents induced within the latter by the inductive field said currents being usually produced by means of a solenoid through which flows an alternating current of industrial periodicity, but said increase of temperature is also due to a great extent to hysteretic cycles or loops.

The intensity of the induced currents and the quantities of heat generated insaid hysteretic cycles are functions of the permeability and of the hysteretic coeficient of the material constituting said mufce. Automatic regulation of temperature is cased, in furnaces of this typaupon the principle that the permeability and hysteresis of the material of the mufile, fall greatly when the temperature of the furnace reaches that of disappearance of the strong magnetism of said material.

According to this invention, it is proposed to make use of the property inherent in ferro-magnetic materials whereby the induction of said materials falls greatly when their temperature reaches and exceeds that at which their strong magnetism disappears.

The ierro-niagnetic inuilie of a furnace constructed in accordance with this invention comprises to this end a heating jacket consisting of a non-magnetic metal of highconductivity or whose strong magnetism disappears at a temperature lower than that which it is proposed to se cure and to maintain within the furnace, within which jacket the alternating induction flux flowing through the ferro=magnetic znuiiie causes in-= duced currents to be set up, said induced currents being a function of the amide inductance and likewise, of course, of the electric characteristics of the conductor circuit, its ohmic resistance, seli inductiou, capacity and the like.

The induced currents thus generated within said heating jacket produce heat therein which in turn heats the furnace mufiie, the ierrounagnetlc material whereof has been so selected that the temperatures at which the disappearance of its strong magnetism begins and ends include withinagnetism occurs.

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in their range the constant temperature which it is desired to secure and to maintain within the furnace. If said induced currents are adequate to generate a quantity of heat greater than the losses of the furnace through radiation and trans- 69 mission, then the temperature of said furnace rises.

The currents set up within the heating jacket depend in the main upon the inductance of said mufile, so long as it has not reached the temperature at which the strong magnetism of the material whereof it is constituted disappears. Thus, when the muli'ie reaches the temperature at which the strong magnetism of its constituent materials disappears, the currents induced in the heating m jacket fall greatly, together with the inductance of said muffle, provided always that the electric characteristics of the inducing circuit remain substantially constant within the range of temperac hire at which said disappearance of the strong {0 If, after disappearance of the strong magnetism of the constituent material of the mufiie, the currents set up in and flowing through said heating jacket are, at that moment, inadequate to generate a quantity of heat equal to the heat lost by the furnace through radiation and transmission, a circumstance which is col lateral to a judicious selection of the electric characteristics of the inducing circuit of said furnace, then the temperature of the muffle falls, passes through that at which its strong magnetism disappears and, upon said strong magnetism reap-= pearing, the inductance of the mufiie rises togather with the heating action of said heating jacket. The temperature of the muiile will therefore attain equilibrium at a value, intermediate between that at which magnetic conversion began and that at which it ended, such that at said temperaturethe inductance of the magnetic core is adequate to generate within the heating jacket induced currents adapted to generate a quantity of heat equal to the caloric losses of the furnace. The automatic regulation of the furnace will then have been secured.

Fig. 1 of the attached drawings exemplifies diagrammatically, without limiting the same, a constructional embodiment of a furnace according to this invention.

Figs. 2 and 3 are charts or diagrams illustrating the automatic thermic action of the furnace.

Within a solenoid 1 through which flows an alternating current is arranged a muiiie 2 of ferromagnetic metal or alloy having suitable crosssection and thickness. The constituent material of said rnufile is so chosen that the temperatures right angle to the axial direction of the winding of solenoid 1, whereas, on the other hand, longi-' tudinal crevices therein are immaterial.

It may furthermore be desirable to close the magnetic circuit outwardly by means of preferably laminated'mases 3 of iron or the like.-

The muiiie is surrounded throughout its eilective height by a heating jacket 4 in contact with or adjacent thereto, adapted to constitute a nonmagnetic conductor at the working temperatures of the mufiie.

The space between solenoid 1 and jacket 4 is filled with a suitable insulating material 5.

The characteristics of the furnace must be so chosen that the quantity of heat generated within heating jacket 4 is greater than the losses of the furnace by'radiation and transmission before the temperature is reached at which magnetic con-' version. occurs, and smaller than said losses after the strong magnetism has entirely'disappeared.

To this end it is necessary to suitably predetermine the voltage and periodicity of the current fed to theterminals of solenoid 1, the mass and nature of magnetic muflie 2, the nature, resistivity and thickness of the conductor jacket 4 and the nature and thickness of the thermic insulator.

As an example, among many, of a constructional embodiment of-a furnace according to this invention, which example does not limit the same and is not a preferred form of construction since a better selection of the characteristics of said furnace would enable a higher degree of efliciency to be attained therein, said furnace may comprise a mume of term-cobalt containing approximately 30% cobalt and 10% iron, together with a small percentage of carbon. The muflie is 12 mm. in thickness and its inner diameter is approximately 150 mm.; 350 mm. of its height is surrounded by a nickel conducto jacket-having a thickness of 3 mm.

Since the temperature of equilibrium of the muflie is 980 C. and that at which the strong magnetism of nickel disappears is in the neighbourhood of 350 0., it may be considered that said nickel jacket is a non-magnetic conductor throughout the range of temperature comprised between 350 C. and 960 0., which is in practice the working range of the furnace.

The inducing circuit is constituted by a winding consisting of four layers of copper ribbon having a width of 10 mm. and a thickness of 3 mm., said layers being insulated from each other by means of braided asbestos and of sheets of mica.

The space enclosed between the inducing winding and the nickel jacket is filled with a suitablecurve of ascending temperature (ordinates) as a function of time (abscissae) is shown by the full line curve ofFig. 2. It will be seen that the temperature of the muffle rises rapidly and besaid cos. c increases rapidly and remains sub .wherein the sole source of heat used in the mufstantially constant adjacent 0.72 within the range extending from 350 C. at which temperature the strong magnetism of nickel disappears, to approximately 950 C., at which temperature the strong magnetism of fem-cobalt begins to disappear.

The above example of an application. of the invention will serve to make clear the advantage which its improvements offer over the process fie consists of the hysteretic phenomena and-the currents induced in said fem-magnetic mufile.

To this end and with the same furnace, it has been sought to secure a like equilibrium of temperature at 960 C. the same period of time, but without the nickel jacket; the mode being heated only by Foucault currents and by hysteretic loops or cycles.

In Fig. 2 the dotted line denotes the curve of variations of temperature as a function of time, and Fig. 3 exhibits the values of cos. at the like moments. p

In order to reach the same equilibrium temperature within the same period of time and hence to obtain the generation of the like quantities of heat within the mufiie, it was necessary to feed a voltage of 165 volts to the terminals of the induction 7 circuit, instead of a voltage of 130 volts. I

The reason for this will become manifest from a comparison of the curves of cos. e, which show that the values of the dotted curve decrease steadily from 0.52 and are considerably lower than the corresponding values of the full-line curve relative to the nickel-jacketed mode.

It may be noted from Fig. 2 that the dotted 12o curve is slightly above the full-line curve, both curves uniting slightly before the temperature of equilibrium. This admits of a simple explanation if it be considered that the cos. of the dotted curve decreases steadily and that it is therefore necessary to supply greater power in order to reach the like temperature within the like periodoi timethaninthecasewherecosremains practically constant.

It will be realized therefore that the process 3130 pursuant to this invention constitutes an improvement upon those form-magnetic mume furnaces which are solely heated by Foucault currents and hysteretic cycles since, even in the case of small muflles, it permits the power factor to be greatly improved.

In order that the regulation of the furnace temperature may be secured with great accuracy, ferro-magnetic bodies must be selected whose inductance changes rapidly as a function of temperature in the neighbourhood of the point of magnetic conversion. It is therefore particularly desirable to make use of form-magnetic bodies, such, for example, as certain ferro-cobalts, whose inductance drops abruptly in the neighbourhood of the temperature of conversion. owing to the inversion of their term-magnetic and etic properties at the said temperature.

If it is desired to secure a predeed distribution of temperature within the furnace, the 159 0 or ring might be surrounded by a heating jacket muflie may be formed in several successive sections consisting of metals or alloys the strong magnetism oi each of which disappears at a different temperature. In this manner, since each section of the muiile has a diflerent temperature of equilibrium, the desired distribution of temperatures may be secured throughout the length or the muifle without it being necessary to change the inductive field at the respective levels. The heating jacket may likewise be divided into sections having diii'erent natures or thicknesses and, in conjunction with a mume similarly divided, this would result in the equilibrium conditions being changed along the latter. The latter alternative would prove useiul when conditions governing cooling or the transmission of heat at a constant temperature vary in the length of the mume.

Furthermore, in the case of a muiiie constructed or different term-magnetic bodies, each such body having varying characteristics and thicknesses and it would be possible by this means to secure in each zone conditions of equilibrium in accordance with the conditions 0! temperature and of cooling initially laid down in regard to the length of said mufiie.

I claim:

1. An induction-heated furnace comprising mume composed of magnetic metal or alloy whose induction falls greatly at the temperature atwhich its strong magnetism disappears, means for generating induced currents therein and a' heating jacket located between said muille and said means, saidjacket being composed of metal which is conductive but non-magnetic at the operating temperature of the furnace.

2. An induction-heated furnace comprising a muiiie composed 9i magnetic metal or alloy whose induction rails/greatly at the temperature at which its strong magnetism disappears, a heating jacket enclosing said muflle, said jacket being composed of metal which is conductive but nonmagnetic at the operating temperature of the lurnace, and means for inducing within said jacket currents such that the heat generated thereby before the magnetic conversion temperature is reached is greater than the losses by radiation and transmission and is less than such losses after the magnetism has disappeared. I

3. An induction-heated furnace as specified in claim 1, wherein the heating jacket is divided into sections of diflerent characteristics and thicknesses.

4. An induction-heated furnace as specified in claim 1 wherein the muiiie is constituted of successive sections of magnetic metals or alloys, the temperature at which the strong magnetism oi each of said sections disappears being individual thereto and diflerent from that of each of the other sections. R

VICTOR JOSEPH CELETIN SORREL. 

